Floor Joist Span Calculator
Calculate maximum permissible spans for UK domestic floor joists in accordance with BS 5268
UK Floor Joist Regulations
Building Standards
All calculations are based on BS 5268-2:2002 and BS 5268-7.1:1990 for structural timber design. These standards establish the permissible stress design methods and span calculation procedures for UK domestic floor joists.
Load Requirements
Standard domestic loadings as per BS 6399-1 for residential floors including furniture, occupants, and floor finishes.
Timber Grades
| Grade | Strength | Common Use |
|---|---|---|
| C16 | 16 N/mm² | Standard construction |
| C24 | 24 N/mm² | Longer spans, higher loads |
Installation Guidelines
Bearing Requirements
- Minimum bearing: 40mm at each end
- Wall thickness: Adequate support from load-bearing walls
- Beam pockets: Properly formed and ventilated
- Damp prevention: DPC under timber bearings
Strutting Requirements
- Spans up to 2.5m: No strutting required
- 2.5m to 4.5m spans: Mid-span strutting
- Over 4.5m spans: Two rows of strutting
- Strutting type: Herringbone or solid blocking
Notching Limits
- Maximum depth: 12.5% of joist depth
- Position: Between 7% and 25% from support
- Holes: Maximum 25% of depth at neutral axis
- Spacing: Minimum 3 diameters between holes
Common Joist Sizes
| Joist Size | 400mm Centres | 450mm Centres | 600mm Centres | Typical Use |
|---|---|---|---|---|
| 47 × 122mm | 2.55m | 2.45m | 2.09m | Small rooms, bathrooms |
| 47 × 147mm | 3.06m | 2.95m | 2.61m | Standard rooms |
| 47 × 170mm | 3.53m | 3.40m | 2.99m | Medium spans |
| 47 × 195mm | 4.04m | 3.89m | 3.39m | Longer spans |
| 47 × 220mm | 4.55m | 4.35m | 3.79m | Large rooms |
Frequently Asked Questions
C16 and C24 refer to strength classes in British Standards. The number indicates the characteristic bending strength in N/mm². C24 timber is approximately 50% stronger than C16, allowing for longer spans with the same joist size or smaller joists for the same span. C16 is more economical and suitable for standard applications, while C24 is used for longer spans or higher loads.
The span is measured as the clear distance between supports, not including the bearing length. For example, if joists rest on walls that are 100mm thick each, and the room is 4m wide, the clear span would be 4m minus the wall thicknesses where the joists bear. Always measure centre-to-centre of supports for accuracy.
The calculator includes standard domestic loads: dead loads (permanent fixtures like flooring, ceiling, services) and imposed loads (furniture, occupants, moveable items). Dead loads typically range from 0.25-1.25 kN/m², while imposed loads are standardised at 1.5 kN/m² for domestic floors. The joist self-weight is automatically included in calculations.
Strutting prevents lateral buckling and distributes loads. It’s required for spans over 2.5m with one row at mid-span for 2.5-4.5m spans, and two rows for spans exceeding 4.5m. Strutting can be herringbone metal struts, solid timber blocking, or proprietary systems. Position strutting at third points for longer spans.
Yes, but with strict limitations. Notches must not exceed 12.5% of joist depth and be positioned between 7-25% of span from supports. Holes should be maximum 25% of joist depth, drilled at the neutral axis (centre line), with minimum 3-diameter spacing between holes. Avoid notches and holes in the same cross-section.
Floor structural work typically requires Building Regulations approval. Submit structural calculations and drawings to your local Building Control department. They may require additional engineer verification for complex layouts, point loads from partitions, or non-standard loadings. Always check before starting work to avoid costly modifications.